1. Field of the Invention
The present invention relates to a method to control a temperature of a laser diode (hereafter denoted as LD), in particular, the invention relates to a method to control a temperature of the LD when a failure occurs in a temperature sensor.
2. Related Prior Art
Various methods to set the optical output from an optical transmitter in stable have been known. Japanese Patent Application published as JP-2005-197984A has disclose an optical transmitter implemented with functions to keep the optical output of the LD in constant even an ambient temperature of the LD varies and to compensate the time degradation of the LD. The optical transmitter disclosed therein provides a look-up table that stores a set of driving currents for the LD each reflecting the temperature dependence of the LD, a differential circuit that compares the optical output of the LD with a reference level, a temperature sensor to sense the ambient temperature, a detector to detect a loss of the input signal, and a processor. The transmitter evaluates the degradation of the LD based on the bias current currently provided thereto, the ambient temperature and the current output of the LD; and may keep the optical output of the LD in constant.
Another prior patent, U.S. Pat. No. 7,056,035, has disclosed another method to maintain the optical output level of the LD and its output wavelength precisely in a long term. The optical transmitter disclosed therein is a type of, what is called as the external modulator, and provides an LD unit an external modulator, a unit of the auto-power control (hereafter denoted as APC) for the LD, an auto-wavelength control (hereafter denoted as AWC), an APC for the modulator, and a controller. The APC-LD unit controls the output of the LD in constant, while the APC-TX unit controls the output of the modulator in constant. The AWC maintains the wavelength of light output form the LD to a preset wavelength by adjusting a temperature of the LD. In a normal mode, the APC-LD control and the AWC control are independently and concurrently carried out as detecting a difference between the output power of the modulator and the preset reference. When this difference becomes larger than a critical level, the target power of the LD is changed.
Still another Japanese Patent Application published as JP-2003-198041A has disclosed a method to prevent an excess current flowing in a thermo-electric controller (hereafter denoted as TEC). The LD module disclosed therein estimates a maximum allowed current supplied to the TEC at which the temperature of the LD becomes less than the allowed highest temperature based on an ambient temperature and the driving current provided to the LD. The controller suppresses the current provided to the TEC less than thus evaluated maximum current.
However, such optical transmitters disclosed in those prior patent require a temperature sensor, typically a thermistor, to sense the temperature of the LD. When a failure occurs in this sensor, or interconnections to the sensor break, the transmitter could not control the optical output of the LD any longer. The present invention provides an optical transmitter able to keep the optical output thereof even when the temperature thereof becomes impossible to be adequately detectable.